This invention relates to the active control of noise generated by an internal combustion engine, and more particularly, to a method of selectively attenuating the amplitudes of multiple harmonic components contained in engine generated noise, based upon the angular rotation of the engine within the operating cycle.
Conventional active noise control systems attenuate undesirable noise, in the form of acoustic waves or mechanical vibrations propagating from a noise source, by producing and superimposing noise canceling waves or vibrations, which are substantially equal in amplitude and frequency content, but shifted 180 degrees in phase with respect to the noise. Recently, this has been achieved through the use of modern digital signal processing and adaptive filtering techniques. Typically, an input sensor, such as a microphone or accelerometer, is used to measure the noise generated by the source, and to develop an input signal for an adaptive filter. This input is transformed by the adaptive filter into an output signal, which drives a speaker or actuator to produce canceling waves or vibrations. An error sensor is employed to measure the observed noise level resulting from the superposition of the original noise and the canceling waves or vibrations, and develops an associated error feedback signal. This feedback signal provides the basis for modifying the parameters of the adaptive filter to minimize the level of the observed noise.
In the past, such systems have been successfully applied to attenuate noise propagating down heating and air ventilating ducts. In these applications, the input sensor is placed upstream in the duct, followed by the cancellation actuator, with the error sensor positioned further downstream. The presence of a feedback path between the input sensor and the cancellation actuator in this type of system requires the use of a recursive type adaptive filter to model the acoustic channel and provide system stability. Although these systems are capable of canceling both repetitive and random noise components, the necessity of a recursive adaptive filtering algorithm, as opposed to the non-recursive type, requires significantly more digital memory and processing time due to the increased computational complexity.
The acoustic and vibrational noise generated by an internal combustion engine differ significantly from that found in heating and air ventilating ducts. The amplitude of engine generated noise can vary quite rapidly with abrupt changes in engine loading, as for example, when the engine is quickly accelerated or decelerated. In addition, engine generated noise is dominated by harmonically related components having frequencies which vary as a function of the engine rotational speed. Also, engines having differing numbers of cylinders generate noise characterized by different dominant harmonic components, due to the different firing frequencies. Finally, acoustic and vibrational noise generated by an engine have different harmonic content, depending upon whether the source of the noise is the air intake system, the exhaust system, or mechanical vibrations produced by operation of the engine.
Consequently, a need exists for a convenient method of selectively attenuating the amplitudes of multiple harmonic noise components generated by internal combustion engines.